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5-40 machine screw

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colin hamilton16/02/2022 20:26:05
140 forum posts
61 photos

So the 40 is the tpi but what is the 5? And where I the UK can I buy a small quantity that dint charge a fortune for shipping? Thanks Colin

JasonB16/02/2022 20:29:04
22744 forum posts
2653 photos
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It's a UNC thread and below 1/4" they use numbers rather than diameter much like wood screws 

What sort of head are you after?

Edited By JasonB on 16/02/2022 20:30:12

Former Member16/02/2022 20:32:37
1085 forum posts

[This posting has been removed]

Mick Dobson16/02/2022 20:34:54
38 forum posts
20 photos

It has diameter 0.125". Often used in US designs.


colin hamilton17/02/2022 12:48:05
140 forum posts
61 photos

Thanks everyone. So below 1/4" they change from and actual dimension to just a number very helpful!!

The head type doesn't matter. I'm about to strip the head of my bridgeport mill. There is part of the quill mechanism which is pulled out after you screw a 5-40 machine screw into it.

Michael Gilligan17/02/2022 13:33:19
20182 forum posts
1053 photos
Posted by colin hamilton on 17/02/2022 12:48:05:


So below 1/4" they change from and actual dimension to just a number very helpful!!


Presumably sarcasm … but why the need ?


Robert Butler17/02/2022 13:46:39
393 forum posts
6 photos


Not sure it was "sarcasm" or just helpful to know.

Robert Butler

colin hamilton17/02/2022 14:01:59
140 forum posts
61 photos

Sarcasm? A bit. Just strikes me as a bit odd they would change the convention. Why not stick with a single system of categorisation?

And what us wrong with a bit of sarcasm.

Andrew Johnston17/02/2022 14:14:12
6601 forum posts
701 photos
Posted by colin hamilton on 17/02/2022 14:01:59:

....what us wrong with a bit of sarcasm.

It can rebound when the originator is wrong. smile

For unified threads below 1/4" the number is related to OD, for numbers from 0 to 12:

OD = thread_number x 0.013 + 0.06


Clive Foster17/02/2022 15:35:48
3135 forum posts
109 photos

I guess switching to numbers avoids silly complicated fractions on smaller sizes.

Same reason for BA being numbered I guess.

Naming and identification systems need to be clear and not easily mis-understood. Not slaves to theoretical rules.

Metric way is nicely tied to rules but in practice it can be less than clear. Decimal points are easily missed or mis-read.

Simple M for standard metric coarse then M diameter ... x ... pitch for fine or constant pitch switches horses in a bad, confusing way.

Should have followed the old American convention of NC, NF, NS diameter ... x ... tpi so MC diameter for standard coarse, MF for standard fine and MS diameter ... x ... pitch for anything non standard such as the various permitted constant pitch sizes.

Much clearer and its obvious when you have a weird one to deal with.


John Purdy17/02/2022 17:35:00
349 forum posts
204 photos


The UNC/UNF series of machine screws run from 0 to 14. #0 is defined as .060" major dia. and each number up increases in major dia. by .013", so #1 is .073", #2 is .086" etc., hence #5 is .125" as said above. Preferred sizes are 0. thru 6, 8, 10, and 12. There is also a series that runs down in size following the same .013" rule to #0000. Can't help with where to get them over there as I live on the other side of the pond. I make my own hex headed ones.


Edited By John Purdy on 17/02/2022 17:36:53

Tony Pratt 117/02/2022 17:47:50
1961 forum posts
12 photos

Several UK suppliers come up on Google, I don't know your exact requirements.


JasonB17/02/2022 18:17:13
22744 forum posts
2653 photos
1 articles

For small amounts try Model fixings, they have Phillips pan heads or Allen head in #5-40

Mick Dobson17/02/2022 18:35:10
38 forum posts
20 photos


if you have trouble sourcing the 5-40 fasteners, there is also the 1/8" x 40 TPI ME (Model Engineer) thread, if this could be incorporated in your application. The ME threads use 55 degrees thread angle rather than the 60 degrees in the case of UNC.

noel shelley17/02/2022 19:31:39
1339 forum posts
21 photos

1/8" whitworth is 40 tpi but 55* though it may fit at this size. Good luck. Noel.

Former Member17/02/2022 19:34:28
1085 forum posts

[This posting has been removed]

JasonB17/02/2022 19:47:16
22744 forum posts
2653 photos
1 articles

It was but I did not want to send the OP there if he had specific head requirements so asked what was needed first.

Former Member17/02/2022 19:48:33
1085 forum posts

[This posting has been removed]

colin hamilton18/02/2022 07:15:39
140 forum posts
61 photos

Thanks everyone.

Naming convention properly understood and fasteners ordered from Model Fixings.

Cheers Colin

Joe McKean18/02/2022 07:30:15
58 forum posts

Always worthwhile having a look at Accu

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